This invention relates to prosthetic grafts for implanting in the vascular system of patients and, in particular, to a porous vascular graft having a removable sheath.
Experiments in the early 1900's established venous and arterial autografting (replacing a section of a patient's blood vessel with a section of vein from elsewhere in the patient) as an effective technique for replacement of damaged or defective blood vessels. However, the need went beyond what could be treated by this technique, leading to a search for artificial or prosthetic veins and arteries for implanting in the vascular system. The need includes not only replacements for veins and arteries but also grafted blood vessels which can withstand repeated puncturing, e.g. for patients undergoing hemodialysis. By the 1950's, at least five characteristics of the ideal vascular graft were identified and became generally accepted. The graft must be pliable, durable, biocompatible, have minimum implantation porosity, and have maximum tissue porosity.
Generally "porosity" refers to whether or not the material is permeable by water. "Implantation porosity" refers to whether or not blood will leak through a graft when circulation is restored to the repaired vessel. "Tissue porosity" refers to the ability of cells from surrounding tissues to infiltrate the walls of the graft. The characteristics of the ideal graft are difficult to achieve simultaneously. Pliability, durability, and bio-compatibility can be achieved by proper choice of material. The last two of the five characteristics, relating to porosity, have proven the most difficult to achieve simultaneously. The porosity of a synthetic graft depends more upon how it was made than upon the material from which it was made.
At present there are three basic techniques for making vascular grafts: knitting, weaving, and stretching or expanding. Knitted or woven grafts are typically made from "Dacron".RTM. and expanded grafts are typically made from PTFE (polytetrafluorethylene or "Teflon".RTM.). Expanded PTFE has a microscopic structure of nodes interconnected by fibrils, and is normally impervious to water. PTFE can be made porous by greatly expanding it but is typically not porous.
There is a problem in that the graft with the greatest tissue porosity also has the greatest implant porosity, i.e. the graft which cells most easily infiltrate, thereby securely attaching the graft to the surrounding tissues, is also the graft which leaks the most blood when circulation is restored. Commercially available expanded grafts do not leak, woven grafts may or may not leak, and knitted grafts will leak. In general, the choice between knitted and woven grafts has been based upon proximity to the heart. More leakage is tolerated the further one is from the heart. Thus, woven grafts are generally used on or near the heart while knitted grafts are generally used away from the heart. The choice between porous and PTFE grafts is generally based upon the more rapid healing with porous grafts versus not having to pre-clot PTFE grafts and the better performance of PTFE grafts in smaller diameters.
Porous grafts, and very porous PTFE grafts, have a disadvantage in the need to control their porosity temporarily at the time of implantation; specifically, to minimize their implantation porosity to reduce leakage. Typically, the graft, which may have been pre-treated with coagulant, is soaked in the patient's blood to form clots in the graft, thereby sealing it. Pre-clotting decreases the pliability of the graft, interrupts the surgery, and extends the surgical time of the patient, all of which are significant disadvantages.
To avoid affecting surgical time, manufacturers of grafts have taken a renewed interest in pre-coated grafts; Cf. Jones et al., "A new sealant for knitted Dacron prostheses: Minimally cross-linked gelatin", Journal of Vascular Surgery, Vol. 7, No. 3, Mar. 1988, pages 414-419. Grafts have been pre-coated with soluble collagen (gelatin), insoluble collagen, albumin, and fibrin, all of which noticeably stiffen the graft. Because the coating stiffens the graft and relies on cell infiltration to re-absorb the coating material, a coating is usually applied only to knitted grafts. Woven grafts, which are already somewhat stiff, are usually not coated by commercial manufacturers.
Coating knitted grafts with organic material raises questions about the purity of the material and the contamination of the graft. The question of contamination has already been raised in connection with uncoated grafts; see, for example, Harris et al. "An In Vitro Study of the Properties Influencing Staphylococcus Epidermidis Adhesion to Prosthetic Vascular Graft Materials", Ann. Surg., November 1987, pages 612-620. This article suggests that a silicone coating reduces the adherence of the S. E. bacterium.
Instead of, or in addition to, coatings, it has also been proposed to use multi-layer grafts of different materials. U.S. Pat. No. 3,105,492 discloses concentric knitted and woven tubes and asserts that this combination need not be externally pre-clotted. U.S. Pat. No. 4,850,999 discloses a knit tube within a braided hose or a PTFE tube within a braided hose. U.S. Pat. No. 4,871,365 discloses a graft having a non-absorbable layer surrounding an absorbable layer, held together by stitches, glue, or frictional contact.
In view of the foregoing, it is therefore an object of the invention to provide a vascular graft which does not require pre-clotting outside the patient.
Another object of the invention is to provide a mechanism for temporarily controlling the porosity of a vascular graft.
A further object of the invention is to render porous vascular grafts non-porous during implantation without decreasing the tissue porosity of the graft.
Another object of the invention is to provide a vascular graft with a non-porous sheath which is easily removed after the graft is surgically implanted.
A further object of the invention is to provide a vascular graft with a sheath which can be removed from a single end, without the need for access to the entire graft.